On the Moon: reflected light and the sun upon the waves
Why a study of the moon must begin from the perspective of mirrors
Under the heading 'On the Moon', Leonardo sets out a programme for studying the moon's essence, insisting it must begin with the optics of plane, concave, and convex mirrors, the nature of the luminous ray, and how it bends through media. He observes that sunlight reflected from sea waves reaches the eye in a pyramidal breadth, only appearing parallel. A sequence of three small margin figures accompanies numbered propositions asking whether the moon keeps a place among its elements, whether it is the lightest of them, and why, if so, it is solid and not transparent.
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A treatise on the essence of the moon begins with optics
Wishing to treat the essence of the moon, Leonardo says one must first describe the perspective of plane, concave, and convex mirrors, define the luminous ray, and establish where a reflected ray is strongest, before the moon itself can be understood.
The sun reflected from sea waves is pyramidal, not parallel
Solar rays striking the sea waves appear to the eye as broad at its angle as at the farthest waves on the horizon, so the reflected splendor is of pyramidal shape and gains breadth with distance, even though to our seeing it looks parallel.
Propositions: is the moon the lightest element, and does it keep a place?
Numbered propositions state that nothing lightest is opaque and nothing lighter lies beneath the less light, then ask whether the moon holds a place among its elements. If it does not fall to the center like the earth, it must be lighter than any element; yet if lightest, why is it solid and not transparent?